Q. How do I use master slides in Google Presentations?

Master slides are a key feature of any presentation slide deck, whether you're using Keynote, PowerPoint or, for this example Google Slides.

Why? Well it's unlikely that whatever template you choose will have the exact layout that you want, maybe the font is wrong, the size, the background, the position of the text box, et cetera. What usually happens is people create a new slide and change it, over and over again, or change, then duplicate, and duplicate that and so on.

Instead of editing the slide you've chosen, you should edit the master version of that slide, like this:

First don't just click the big + to add a slide, click the little drop down menu next to it, and choose the master slide you like:

Now to tweak that, don't change the actual slide, instead change the master slide, go to Slide > Edit master. Now make the design tweaks, and it will update. Now whenever you choose that slide from the slide picker it will look exactly the way you want it to look, here's an example:

Change 1 Slide, change all...

But what's even better, it's very likely that you will want to reconsider your slide layout, but as you've been working with the master slides, you can change every slide you based on that master, all in one go. Just choose any of the slides that used the same slide layout, go to Slide > Master, and now watch in amazement, as every tweak you make, cascades down all of the slides it was used for—amazing!